The invention relates a process for depolymerizing condensation polymers. More specifically, the invention is directed to a process for depolymerizing condensation polymers in which a condensation polymer is subjected to a pre-molecular weight reduction step and thereafter subjected to neutral hydrolysis.
The continuous hydrolysis of high molecular weight condensation polymers including polyesters such as polyethylene terephthalate, polyamides such as nylon 6 and nylon 66 and polycarbonates has been disclosed in the art by references including Canadian Pat. No. 611,032 to Katzschmann; Japanese SHO No. 49 [1974] - 76968 to Tanaka et al; and in co-pending U.S. application Ser. No. 563,812, filed Dec. 21, 1983. These processes involve treating a condensation polymer, including oligomeric waste, with a substantial excess of water under conditions of high temperature and pressure and result in the reversal of polycondensation. For example, terephthalic acid and ethylene glycol are the primary products of polyethylene terephthalate hydrolysis.
Under optimum hydrolysis conditions of high pressure, high temperature and high polymer concentration, depolymerization products such as terephthalate acid, exhibit a high degree of corrosiveness on reactor components. Thus, in those parts of the reaction vessels that are exposed to extreme conditions, expensive acid-resistant metals are preferred, e.g., titanium, Hastaloys, Carpenter Stainless and the like. To minimize the cost of these parts, in particular the hydrolyzer vessel itself, a small vessel is preferred. Since smaller vessels dictate high throughputs of feed material, processes have been sought to minimize residence time of the condensation polymer in the hydrolyzer but which nevertheless effect substantially complete depolymerization, e.g., of polyethylene terephthalate to terephthalic acid and ethylene glycol.
Several factors are believed to govern the rate of hydrolysis. These include pressure (or temperature) in the hydrolyzer, surface-to-volume ratio of the condensation polymer feed materials, the degree of crystallinity of the polymer feed material, and the molecular weight of the polymer feed material. Hydrolysis rates may be increased, resulting in reduced residence by: (1) increasing pressure (or temperature); (2) increasing the surface to volume ratio of the feed material; (3) decreasing the degree of crystallinity of the feed material; or (4) decreasing the molecular weight of the feed material. In actual practice, there are practical ceilings of operation. Pressure is limited by equipment duty, availability and safety. Surface/volume ratios and degrees of crystallization are usually fixed prior to hydrolysis by the physical form and nature of the polymer, e.g., polyethylene terephthalate, being feed. Thus, the artisan is faced with a limited degree of flexibility in modifying hydrolysis processes to increase reaction rate within the hydrolyzer.